Heating, ventilation, and air conditioning systems (HVAC systems) sometimes incorporate gas furnaces for providing a heating effect to temperature controlled areas or comfort zones. Some gas furnaces comprise draft inducers that pull flue gases resulting from combustion through heat exchangers. It is known that draft inducers cannot dependably be factory set to a particular speed or flowrate in a manner that accommodates for the wide variation of installation furnace configurations and transient pressure fluctuations that may be present amongst different installation locations. For example, some gas furnaces may be installed with substantially different lengths of piping connected to an exhaust vent. Accordingly, it is known to provide a furnace with a variable speed draft inducer, the speed or flowrate of which may be adjusted once the gas furnace is installed and/or in operation. Some gas furnaces provide systems configured to learn operating speeds that are suitable for a particular installation of a gas furnace. For example, U.S. Pat. No. 6,257,870 (referred to hereinafter as the '870 patent) and U.S. Pat. No. 5,791,332 disclose systems and methods for operating a variable speed draft inducer of a gas furnace to account for static and dynamic variations in heat exchanger pressure differential, HXΔP.
In some systems, operation of a gas furnace may be predicated upon feedback from a plurality of switches and/or sensors. For example, in some gas furnaces, a combustion system may be halted from operation when one or more of a temperature limit sensor, a pressure switch, and a gas valve relay are in states inconsistent with safe operation. Specifically, if a temperature limit sensor reports that a temperature is too high the combustion system may be turned off. Similarly, if a pressure switch that ensures a safe amount of exhaust flow reports that exhaust flow is not sufficient, the combustion system may be turned off. Further, if a gas valve relay is inappropriately in an open state, the combustion system may be turned off. In some gas furnaces, the above methods of ensuring safe operation of a gas furnace may be sufficient and/or required.